Both shared and specialized spinal circuitry for scratching and swimming in turtles.

In principle, nervous systems could generate a behavior either via neurons that are relatively specialized for producing one behavior or via multifunctional neurons that are shared among multiple, diverse behaviors. I recorded extracellularly from individual turtle spinal cord neurons while evoking hindlimb scratching, swimming, and withdrawal motor patterns. The majority of spinal neurons recorded were activated during both scratching and swimming motor patterns, consistent with the existence of shared circuitry for these types of limb movements. These neurons tended to have a similar degree of rhythmic modulation of their firing rate and a similar phase preference within the hip flexor activity cycle during scratching and swimming motor patterns. In addition, a substantial minority of neurons were activated during scratching motor patterns but silenced during swimming motor patterns. This raises the possibility that inhibitory interactions between some scratching and swimming neural circuitry play a role in motor pattern selection. These scratch-specialized neurons were also less likely than the putative shared neurons to be activated during withdrawal motor patterns. Thus, these neurons may represent two separate classes, one of which is used generally for hindlimb motor control and the other of which is relatively specialized for a subset of hindlimb movement types.